The COVID-19 pandemic has been characterized by multiple waves of infection with different SARS-CoV-2 variants of concern (VOC). The Omicron variant generally causes milder symptoms than other VOC, especially in vaccinated individuals, and is highly transmissible — leading many to wonder whether immunity induced by Omicron may provide for cross-variant protection and bring us closer to the end of the pandemic. However, a report in Nature now shows that Omicron only induces a limited humoral immune response and little cross-variant neutralization in unvaccinated individuals. By contrast, Omicron breakthrough infections in vaccinated individuals induced high titres of neutralizing antibodies against all VOC. This indicates that although Omicron infection acts as a ‘booster’ in vaccinated individuals, it provides little protection from other VOC in unvaccinated individuals.